Prior to the availability of lumbar arthroplasty, interbody fusion was the primary treatment modality for symptomatic lumbar disc disease. This procedure could typically remove the pain generator from the disc, but results could also include at least some loss in the ability to absorb shock, as well as a reduction in the range of motion of the disc. As such, additional stress could be experienced and lead to further premature degeneration of the adjacent levels of the spinal segment.
Lumbar arthroplasty through the anterior approach has become well established as an alternative treatment modality for symptomatic lumbar disc disease. This allows the removal of the lumbar disc and restoration of normal load transmission through the disc, in addition to allowing preservation of motion in the treated segment. The main aims of lumbar arthroplasty are the removal of the pain generator, the restoration of normal loading across the disc, the maintenance of motion of the segment and therefore, the reduction of strain on the adjacent levels.
Each motion segment of the spine moves around an instantaneous center of rotation. The instantaneous axis of rotation is defined as the axis perpendicular to the plane of motion passing through a point in the vertebral body that does not move. For each spinal motion segment, this point is essentially the point about which the motion segment rotates, and is also referred to as the center of rotation. If the spine is altered in any way, such as with disc and facet degeneration or spinal implantation, this center of rotation shifts. In a normal disc this center of rotation moves during flexion-extension to form an ellipse, and is located next to the upper endplate of the lower vertebra in the posterior third of the disc. With the onset of degeneration, this center of rotation may shift in an unpredictable manner. Thus, a goal of spinal arthroplasty includes restoration of the axis of rotation as closely as possible to that of a natural, healthy spinal segment.
Following a disc replacement, it would be beneficial for the center of rotation of the artificial disc to approximate that of the normal disc. Matching the center of rotation could result in the normal movement of the facet joints without undue strain and with efficient load transfer at the operated and adjacent levels. On the other hand, if the center of rotation of the disc prosthesis is not similar to that of a normal, healthy segment, the facet joints may experience additional strain during movement, which could result in increased pain, hypertrophy and/or fusion. The adjacent levels may also experience additional strain due to a non-synchronous movement of the operated motion segment, which could further contribute to accelerated degeneration at adjacent levels of the spinal segment.
A posterior dynamic stabilization system may be provided in a spinal segment to provide some degree of controlled multiplanar motion. The primary motions of a spinal segment include flexion and extension having a center of rotation near the anatomic normal, as well as rotation and side bending to a lesser degree.
Posterior dynamic fixation devices are commonly fixed to the pedicles at both ends and allow some unconstrained motion, which is primarily linear motion of flexion and extension. These devices are typically used as an alternative to spinal fusion in the treatment of spinal stenosis and other pathologies, as they may provide a stabilizer for certain abnormal motions. They may also be used to stabilize a motion segment adjacent to a spinal fusion. However, as discussed above, if a device has a center of rotation different than that of a normal, healthy disc, it could result in abnormal load transfers to the adjacent segment and subsequent non-physiological movement at the treated segment.
With the development of posterior lumbar disc arthroplasty methods, this center of rotation becomes even more crucial. This is due to the prevailing trend that most posterior lumbar arthroplasty procedures may require a significant resection of the facet joints for insertion, leading to a stronger need for a posterior dynamic stabilizer device. If the centers of rotation of the disc arthroplasty and the posterior dynamic device are dissimilar, the resulting range of motion of the spinal segment may be substantially reduced and/or eliminated.
Accordingly, it would be desirable to provide a posterior lumbar disc replacement and/or a posterior dynamic stabilization device or facet replacement system which could be positioned to provide movement about a center of rotation substantially similar to that of a healthy, intact intervertebral disc. It would further be desirable to provide a posterior dynamic stabilization system which can continuously adjust to a moving center of rotation of the disc replacement, or, if used on its own, with that of the lumbar disc of that level.